Alkali corrosion resistant coatings for Si3N4 ceramics

The use of ceramic oxide coatings on silicon nitride is one method to improve its alkali corrosion resistance. Four oxide coatings, including (Ca_0.6, Mg_0.4) Zr₄(PO₄)₆ (CMZP), zirconia, mullite and alumina, were examined. These coatings were applied on Si₃N₄ using both sol–gel and dip coating techniques. The coated and uncoated samples were exposed to sodium molten-salt and sodium-containing atmospheres at 1000 °C for 50 h. The weight loss of all the coated samples was less than that of the uncoated Si₃N₄ with CMZP-coated samples exhibiting the smallest weight loss. There was no decrease in the flexural strength of Si₃N₄ after coating with zirconia and CMZP, and a decrease in strength after coating with either mullite or alumina. After alkali exposure, the strength of the CMZP and zirconia coated samples were significantly higher than those of the mullite-coated, alumina-coated, and uncoated Si₃N₄. The observed behaviour is explained in terms of the microstructure and protection mechanisms. This revised version was published online in November 2006 with corrections to the Cover Date.     

Synthesis and sintering of Ca0.5Zr2P3O12 -a low thermal expansion material

The single phase compound Ca_0.5Zr₂P₃O₁₂ (CZP) was prepared by solid state reaction technique. This material shows a negative thermal expansion in the temperature region of 30°–500°C. The effect of MgO and ZnO addition on the sintering behavior and thermal expansion characteristics of Ca_0.5Zr₂P₃O₁₂ was investigated. Mg₃(PO₄)₂ and Zn₃(PO₄)₂ were observed as minor phases responsible for improving the overall thermal expansion of CZP + MgO, ZnO systems. SEM studies and density data are also discussed. Observed sintering kinetics suggest that a liquid phase is promoting the sintering reaction. 98+% of theoretical density and near zero expansion behavior in certain compositions were observed.     

Preparation of (NH4)Zr2(PO4)3 and HZr2(PO4)3

(NH₄)Zr₂(PO₄)₃ has been prepared, hydrothermally, from α-zirconium phosphate in three different ways; (1) from amine intercalates at 300°C, (2) from mixtures of ZrOCl₂·8H₂O in excess (NH₄)H₂PO₄ and (3) reaction of NH₄Cl with Zr(NaPO₄)₂. Ammonium dizirconium triphosphate is rhombohedral with a = 8.676(1) and $\text{c}\text{= 24.288(5)}\text{A}\text{?}$. It decomposed on heating to HZr₂(PO₄)₃. Below 600°C a complex, as yet unindexed, X-ray pattern was obtained. A very similar X-ray pattern was obtained by washing LiTi_0.1Zr_1.9(PO₄)₃ with 0.3N HCl. Heating this phase or NH₄Zr₂(PO₄)₃, above 600°C resulted in the appearance of a rhombohedral phase of HZr₂(PO₄)₃ with cell dimensions a = 8.803(5) and $\text{c}\text{= 23.23(1)}\text{A}\text{?}$. The protons were not completely removed until about 1150°C. Decomposition of (NH₄)Zr₂(PO₄)₃ at 450°C yielded an acidic gas whereas at 700°C NH₃ was evolved. A possible explanation for this behavior is presented.     

Structural features of AgCaCdMg2(PO4)3 and AgCd2Mg2(PO4)3, two new compounds with the alluaudite-type structure, and their catalytic activity in butan-2-ol conversion

AgCaCdMg₂(PO₄)₃ and AgCd₂Mg₂(PO₄)₃, two new compounds with the alluaudite-type structure, were synthesized by a solid state reaction in air at 750 °C. The X-ray powder diffraction pattern of AgCaCdMg₂(PO₄)₃ indicates the presence of small amounts of (Ca, Mg)₃(PO₄)₂ with the whitlockite structure, as impurity, whereas AgCd₂Mg₂(PO₄)₃ is constituted by pure alluaudite. The Rietveld refinements of the X-ray powder diffraction patterns indicate an ordered cationic distribution for AgCd₂Mg₂(PO₄)₃, with Ag on A(2)′, Cd on A(1) and M(1), and Mg on M(2), whereas a disordered distribution of Cd and Ca between the A(1) and M(1) sites is observed for AgCaCdMg₂(PO₄)₃. The catalytic properties of these compounds has been measured in reaction of butan-2-ol dehydrogenation. In the absence of oxygen, both samples exhibit poor dehydrogenation activity. All samples displayed no dehydration activity. Introduction of oxygen into the feed changed totally the catalytic behavior of the catalysts. The production of methyl ethyl ketone increases with time on stream and the reaction temperature. AgCaCdMg₂(PO₄)₃ is more efficient than AgCd₂Mg₂(PO₄)₃.     

On the proton conductor (H3O)Zr2(PO4)3

The compound HZr₂(PO₄)₃ was converted to (H₃O)Zr₂(PO₄)₃ by refluxing in water for 12 or more hours. The water is lost above 150°C to regenerate the original triphosphate. The hydronium ion phase is rhombohedral with hexagonal axes of a = 8.760(1) and $\text{c}\text{= 23.774(4)}\text{A}\text{?}$. Proton conduction in these compounds was investigated by an ac impedance method over the frequency range 5Hz – 10MHz. The activation energy for (H₃O)Zr₂(PO₄)₃ in the temperature range of 25 to 150°C was 0.56eV while the corresponding value for HZr₂(PO₄)₃ (125 – 300°C) was 0.44eV.     

Influence of Ca0.8Sr0.2TiO3 on the microstructures and microwave dielectric properties of Nd(Mg0.4Zn0.1Sn0.5)O3 ceramics

The influence of Ca_0.8Sr_0.2TiO₃ on the microstructures and microwave dielectric properties of Nd(Mg_0.4Zn_0.1Sn_0.5)O₃ ceramics were investigated by the conventional solid-state method. The X-ray diffraction peaks of (1 − x)Nd(Mg_0.4Zn_0.1Sn_0.5)O₃–xCa_0.8Sr_0.2TiO₃ ceramic system shifted to higher angles as x increased. The dielectric constant increased from 31.8 to 47.7, the quality factor (Q × f) decreased from 54,200 to 42,800 GHz, and the temperature coefficient of resonant frequency (τ _f ) increased from −43 to +41 ppm/°C as x increased from 0.5 to 0.7 when (1 − x)Nd(Mg_0.4Zn_0.1Sn_0.5)O₃–xCa_0.8Sr_0.2TiO₃ ceramic system sintered at 1,600 °C for 4 h.     

Luminescent properties of Eu-activated nanophosphors Ca3Y0.8Gd0.2(VO4)2.4(PO4)0.6: Eu for light-emitting diodes

Eu³⁺-doped Ca₃Y_0.8Gd_0.2(VO₄)_2.4(PO₄)_0.6 nanophosphors have been prepared by modified solid-state reaction. X-ray powder diffraction, transmission electron microscopy (TEM), photoluminescence excitation and emission spectra were used to characterize the resulting samples. X-ray powder diffraction (XRD) analysis confirmed the formation of YVO₄. Photoluminescence (PL) results showed that the phosphor could be efficiently excited by UV-visible light from 350 to 550 nm, exhibiting bright orange-red emission(excited by 397) and red emission(excited by 467), which has potential application as a phosphor for UV and blue GaN-based light-emitting diodes (LEDs). TEM images show that the grain size of Ca₃Y_0.45Eu_0.35Gd_0.2(VO₄)_2.4(PO₄)_0.6 is about 39 nm, which is in full agreement with the theoretical calculation data from the XRD patterns.     

Crystal structure and luminescent properties of europium-activated Ca10? x M x (PO4)6F2 (M = Pb, Mg) prepared via precipitation from aqueous solutions

Phase-pure polycrystalline fluorapatites with the general formula Ca_10−x M _x (PO₄)₆F₂:Eu³⁺(M = Pb, Mg) have been prepared by precipitation from aqueous solutions, and the effects of the Pb²⁺ and Mg²⁺ ions, differing markedly in ionic radius, on the structure, morphology, and luminescence spectra of the fluorapatites have been investigated. The Pb²⁺ and Mg²⁺ contents are shown to influence the Eu³⁺ distribution over inequivalent sites in the crystal structure of Ca_10−x M _x (PO₄)₆F₂. Original Russian Text ? N.V. Babayevskaya, Yu.N. Savvin, A.V. Tolmachev, 2007, published in Neorganicheskie Materialy, 2007, Vol. 43, No. 8, pp. 976–980.     

Synthesis and luminescence properties of Eu2+-activated Ca4Mg5(PO4)6 for blue-emitting phosphor

Ca ₄ Mg ₅ (PO ₄ ) ₆ :Eu²⁺^{\boldsymbol{2+}} blue-emitting phosphor was synthesized by the combustion-assisted synthesis method under reductive atmosphere. The products were characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM) and photoluminescence (PL) spectrum. XRD analysis confirmed the formation of Ca ₄ Mg ₅ (PO ₄ ) ₆ pure phase. Photoluminescence results showed that the phosphor can be excited efficiently by UV light range from 230–400 nm, and then exhibited bright blue light with peak wavelength at 431 nm. It is a very promising candidate as a blue-emitting phosphor for potential applications in display devices.     

Diffuse transition and piezoelectric properties of Pb[(Zr1−xTix)0.74(Mg1/3Nb2/3)0.20(Zn1/3Nb2/3)0.06]O3 Ceramics

In this work, the piezoelectric ceramic system of Pb[(Zr_1−xTi_x)_0.74(Mg_1/3Nb_2/3)_0.20(Zn_1/3Nb_2/3)_0.06]O₃, 0.47≤x≤0.57, with composition close to the morphotropic phase boundary, was studied. From the results of X-ray diffraction and piezoelectric measurement, ceramics near x=0.51 were found at the morphotropic phase boundary (MPB) between the tetragonal and pseudocubic perovskite. The planar coupling factor (k_p=0.72) is high at compositions near the MPB, but the mechanical quality factor (Q_m=75) is low. The calculation of the diffuseness of phase transition shows that the region of phase coexistence of this system is broader than that of the ternary system.     

The cation distribution between five- and six-coordinated sites in some (Mg,Me)3(PO4)2) solid solutions

Approximate homogeneity ranges at about 1073 K have been determined for some (Mg,$\text{Me}$)₃(PO₄)₂ solid solutions. X-ray powder diffraction data are given and the observed changes in unit cell dimensions are discussed. The Mg₃(PO₄)₂ structure, isotypic with γ-Zn₃(PO₄)₂, contains five- and six-coordinated cation sites, M1 and M2 respectively. The M1 site preference order is Zn²⁺ > Co²⁺ > Fe²⁺ > Mg²⁺ > Mn²⁺.     

Effect of ZrO2 addition on crystallization behaviour, porosity and chemical-mechanical properties of a CaO-TiO2-P2O5 microporous glass ceramic

2-6 mol% ZrO₂ was added to a base glass composition of P₂O₅ 31.25, CaO 43.75, TiO₂ 25 (mol%) at the expense of TiO₂. The prepared glasses were crystallized to bulk glass ceramics containing the major phases of β-Ca₃(PO₄)₂ and CaTi₄(PO₄)₆. DTA was utilized to determine the appropriate phase separation-nucleation and crystallization temperatures. The crystalline products and resulting microstructures were examined by XRD and SEM. The β-Ca₃(PO₄)₂ phase was dissolved out by leaching the resulting glass ceramics in HCl, leaving a porous skeleton of CaTi₄(PO₄)₆. It was shown that ZrO₂ addition resulted in reduction of volume porosity and mean pore diameter while the specific surface area was increased. The smallest median pore diameter and largest surface area were 8.6 nm and 32 m² g⁻¹ respectively obtained for the specimen containing 6 mol% ZrO₂. The ZrO₂ addition also improved the chemical durability and bending strength of porous glass ceramics.     

Low thermal expansion and low thermal expansion anisotropy ceramic of Sr0.5Zr2(PO4)3 system

A low thermal expansion ceramic with a very low thermal expansion anisotropy was synthesized from the Sr_0.5Zr₂(PO₄)₃ system. The sintering was promoted by addition of MgO, and the solgel technique also improved the sinterability. The thermal expansion of the crystal was lowered by substituting Nb⁵⁺ for Zr⁴⁺ and 1/2Sr²⁺ pairs, becoming near-zero for Sr_0.25Nb_0.5Zr_1.5(PO₄)₃. All dense ceramics in this system had a strength of about 80 MPa, and did not suffer microcracking even in the coarse-grained polycrystalline ceramics, owing to the very low thermal expansion anisotropy of the crystals.     

Low temperature sintering and properties of Ca–Al–B–Si–O glass/ceramic composites with various ceramic fillers

Low-temperature sintering and properties of low temperature co-fired ceramics materials based on a typical Ca–Al–B–Si–O glass and various ceramic fillers such as (Zr_0.8Sn_0.2)TiO₄, (Ca_0.5Mg_0.5)TiO₃, BaSm₂Ti₄O₁₂ and CaTiO₃ were investigated. Densification, crystallization and dielectric properties are found to strongly depend on the type of filler. The densification process of glass/ceramic composites with different ceramic fillers is mainly from 600 to 925 °C, and the initial compacting temperature of samples is 600 °C. The initial rapid densification of samples starts after glass softening temperature of samples. The XRD patterns of (Ca_0.5Mg_0.5)TiO₃ and CaTiO₃ samples demonstrate crystalline phases, CaTiO(SiO₄) and CaTiSiO₅, respectively, as a result of firing at 875 °C for 15 min. The high dielectric constant fillers produce high ε_r values of the dielectric samples. The maximum dielectric constant of samples for (Zr_0.8Sn_0.2)TiO₄, (Ca_0.5Mg_0.5)TiO₃, BaSm₂Ti₄O₁₂ and CaTiO₃ filler is 14.02, 16.21, 18.64 and 23.78, respectively. Comparing with other samples, the specimens for (Ca_0.5Mg_0.5)TiO₃ and CaTiO₃ ceramic filler have lower dielectric loss. Especially, the sample for (Ca_0.5Mg_0.5)TiO₃ filler exhibits the lowest dielectric loss of 0.00011.     

In vitro corrosion resistance and antibacterial performance of novel tin dioxide-doped calcium phosphate coating on degradable Mg-1Li-1Ca alloy

A SnO_2-doped calcium phosphate(Ca-P-Sn) coating was constructed on Mg-1 Li-1 Ca alloy by a hydrothermal process. The fabricated functional coatings were investigated using scanning electron microscopy(SEM), X-ray diffraction(XRD) and Fourier transform infrared spectroscopy(FT-IR). A triple-layered structure, which is composed of Ca_3(PO_4)_2,(Ca, Mg)_3(PO_4)_2, SnO_2, and MgHPO_4·3 H_2O, is evident and leads to the formation of Ca_(10)(PO_4)_6(OH)_2 in Hank's solution. Electrochemical measurements, hydrogen evolution tests and plating counts reveal that the corrosion resistance and antibacterial activity were improved through the coating treatment. The embedded SnO_2 nanoparticles enhanced crystallisation of the coating.The formation and degradation mechanisms of the coating were discussed.     

Influence of ZnO additions to 0.8(Mg0.95Co0.05)TiO3–0.2Ca0.6La0.8/3TiO3 ceramics on sintering behavior and microwave dielectric properties

The effects of ZnO addition on the microstructures and microwave dielectric properties of 0.8(Mg_0.95Co_0.05)TiO3–0.2Ca_0.6La_0.8/3TiO₃ ceramics were investigated. ZnO was selected as liquid phase sintering aids to lower the sintering temperature of 0.8(Mg_0.95Co_0.05)TiO3–0.2Ca_0.6La_0.8/3TiO₃ ceramics. With ZnO additives, the densification temperature of 0.8(Mg_0.95Co_0.05)TiO₃–0.2Ca_0.6La_0.8/3TiO₃ can be effectively reduced from 1450 to 1200–1325 °C. The crystalline phase exhibited no phase difference at low addition levels (0.25–2 wt.%). It is found that low-level doping of ZnO (0.25–2 wt.%) can significantly improve the density and dielectric properties of 0.8(Mg_0.95Co_0.05)TiO₃–0.2Ca_0.6La_0.8/3TiO₃ ceramics. The quality factors Q × f were strongly dependent upon the amount of additives. Q × f values of 36 000 and 13 000 GHz could be obtained at 1200–1325 °C with 1 and 2 wt.% ZnO additives, respectively. During all additives ranges, the relative dielectric constants were significantly different and ranged from 23.1 to 27.96. The temperature coefficient varies from 14.1–24.3 ppm/°C.     

Corrosion resistance property of micro–arc oxidation coatings on Mg–Li alloy obtained in different systems

通过三种优化工艺体系在Mg--5%Li合金表面上生长陶瓷膜层, 分析了膜层的厚度、显微结构、相组成和耐蚀性. 结果表明, 三种膜层都含有MgO相, 微弧氧化试样的耐蚀性能都明显提高. 使用Na₃PO₄体系制备的膜层含有MgF₂, 膜层最厚、表面有大量裂纹; 使用Na₂SiO₃体系制备的膜层含有橄榄石型Mg₂SiO₄, 耐点蚀性能最好; 使用Na₂SiO₃--Na₃PO₄体系制备的膜层含有MgSiO₃, 致密性最好, 膜层耐均匀腐蚀性能最好.     

Americium and plutonium in trigonal phosphates (NZP type) Am1/3[Zr2(PO4)3] and Pu1/4[Zr2(PO4)3]

Phosphates Am_1/3[Zr₂(PO₄)₃] and Pu_1/4[Zr₂(PO₄)₃] were synthesized and studied by X-ray diffraction. The X-ray patterns were indexed in trigonal crystal system, and the unit cell parameters were evaluated. The possibility of accommodation of actinides in the framework cavities of the structure of sodium zirconium phosphate was demonstrated for the first time. It was suggested that incorporation of highly charged Pu and Am cations in the framework cavities decreases the symmetry of the crystal structure to simple trigonal. The rates of Pu leaching from a ceramic material based on phosphate Pu_1/4[Zr₂(PO₄)₃] were measured.     

Fabrication of NaZr<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>-type ceramic materials by spark plasma sintering

Ceramic materials based on Ca_0.5Zr₂(PO₄)₃ and NaFeNb(PO₄)₃, structural analogs of NaZr₂(PO₄)₃ (NZP), were prepared by spark plasma sintering. At sintering temperatures of 1100–1200 and 880°C and sintering times of 12 and 3 min, the relative densities reached were 99.1 and 99.9%, respectively. According to X-ray diffraction data, the sintering process caused no changes in phase composition. The ceramics had a dense, homogeneous microstructure and ranged in grain size from 0.5 to 2.5 μm.     

Influence of doping Nb5+ and Mn2+ on the PTCR effects of Ba0.92Ca0.05(Bi0.5Na0.5)0.03TiO3 ceramics

As a positive temperature coefficient of resistivity (PTCR) material, Ba_0.92Ca_0.05(Bi_0.5Na_0.5)_0.03TiO₃ ceramics with donor doping of Nb⁵⁺ and acceptor doping of Mn²⁺ were prepared by a conventional mixed oxide method. The influence of contents of Nb⁵⁺ and Mn²⁺ on the microstructure and PTCR characteristics of Ba_0.92Ca_0.05(Bi_0.5Na_0.5)_0.03TiO₃ ceramics sintered at 1,360°C for 2 h was investigated. The result showed that the Curie temperature (T _c) was shifted to a lower temperature with increasing of the content of Nb⁵⁺ and the resistance jump (ρ_max/ρ_min) was enhanced with doping of Mn²⁺. The grain size of ceramic sample decreased with increasing of contents of donor Nb⁵⁺ and acceptor Mn²⁺. The Ba_0.92Ca_0.05(Bi_0.5Na_0.5)_0.03TiO₃ ceramic with 0.4 mol%Nb⁵⁺ and 0.04 mol%Mn²⁺ exhibited a low ρ_RT of 5.0 × 102 Ω cm, a typical PTCR effect of ρ_max/ρ_min > 10³, and a T _c of 158°C.